This present application relates generally to methods, systems, and apparatus for clearance between a stationary component and a movable component of a rotating machine. More specifically, but not by way of limitation, the present application relates to methods, systems, and apparatus pertaining to measuring blade tip clearance between rotating turbine blades and surrounding stationary structures in combustion turbine engines, which, as used herein and unless specifically stated otherwise, is meant to include all types of turbine or rotary engines, including gas turbine engines, aircraft engines, steam turbine engines.
A gas turbine engine (which, as discussed below, may be used to illustrate an exemplary application of the current invention) includes a compressor, a combustor, and a turbine. The compressor and turbine generally include rows of blades that are axially stacked in stages. Each stage includes a row of circumferentially-spaced stator blades, which are fixed, and a row of rotor blades, which rotate about a central axis or shaft. In operation, generally, the compressor rotor blades rotate about the shaft, and, acting in concert with the stator blades, compress a flow of air. The supply of compressed air then is used in the combustor to combust a supply of fuel. Then, the resulting flow of hot expanding gases from the combustion, i.e., the working fluid, is expanded through the turbine section of the engine. The flow of working fluid through the turbine induces the rotor blades to rotate. The rotor blades are connected to a central shaft such that the rotation of the rotor blades rotates the shaft. In this manner, the energy contained in the fuel is converted into the mechanical energy of the rotating shaft, which, for example, may be used to rotate the rotor blades of the compressor, such that the supply of compressed air needed for combustion is produced, and the coils of a generator, such that electrical power is generated. During operation, because of the extreme temperatures of the hot-gas path, the velocity of the working fluid, and the rotational velocity of the engine, rotor blades become highly stressed with extreme mechanical and thermal loads.
One of ordinary skill in the art will appreciate that the efficiency of gas turbine engines is significantly impacted by clearance between the outer radial tip of the rotor blades and the surrounding stationary structure, which is referred to herein as “tip clearance”. It will be appreciated that tighter clearances decrease the leakage flow around the rotor blades, which improves engine efficiency. Tighter tip clearances, though, increase the risk that rotating parts will make contact with or rub against non-rotating parts during one of the engine's several operational modes, particularly considering the fact that tip clearances generally vary based upon operating conditions. Primarily, this is due to the different thermal expansion characteristics of many of the engine components. Of course, having rotating and stationary parts rub or make contact during operation is highly undesirable because it can cause extensive damage to the engine or failure of certain components. In addition, rubbing may result in increased clearances once the event that caused the rubbing passes. On the other hand, the engine may be designed with looser clearances that decrease the likelihood of rubbing parts. However, this is undesirable because it generally allows for more leakage and, thereby, decreases the efficiency of the engine.
Many newer gas turbines employ active clearance control systems to manage the clearance during a myriad of operating conditions so that a tight, non-rubbing clearance is maintained. It will be appreciated that these systems need regular, updated, and accurate tip clearance data to realize the full benefit of the clearance control system. Conventional measurement systems measure tip clearance with proximity sensors positioned in the hot-gas path. Typically, these probes are positioned directly over the rotor blades and measure the distance between the probe and the blade tips of the rotor blades as the blades pass.
The downside of positioning the sensors in this manner is that the sensors are exposed to the extreme temperatures of the hot gas flowpath. Sensors that are able to withstand these conditions while providing accurate measurements are expensive. Even so, because of the extreme conditions of the hot-gas path, these sensors have short life spans, which increases costs and maintenance requirements. Also, these sensors typically require a supply of cooling air, which may be bled from the compressor or supplied from an auxiliary source. It will be appreciated that providing cooling air in this manner adds complexity to engine systems and, because the air supplied for cooling purposes reduces the supply of air that may be used for combustion, decreases the efficiency of the engine.
As a result, there is a need for improved apparatus, methods and systems relating to the cost-effective systems that accurately calculate and monitor tip clearances in turbine engines.